Producing Fine Particles for Multiple Applications

The market demand for finer particles is growing and so process industries are exploring grinding technologies that are capable of higher maximum yields and throughput. Cryogenic grinding solutions have historically been used for specialty or hard-to-grind and heat sensitive materials.

Testing services allow the manufacturer to compare grinding technology systems to determine the best solution for a particular product. Under laboratory conditions, it is possible to test whether the required production rates and particle size distribution can be achieved.

However, recent advances mean that they can be employed to create ultra-fine particles used for the manufacture of high-performance polymers and plastics.

Cryogenic grinding technology has been used in the waste recycling industry to produce rubber crumb since the early 1990s. The main advantage is that when liquid nitrogen (LIN) is used to cool the material to cryogenic temperatures, it becomes more brittle. This means that less energy can be used to easily break it into small particles. The same technology is used in the production of plastics that have a variety of uses including making plastisols, powder coatings, carpet backing and textile coatings.

Typically, in the plastics industry, cryogenic grinding is used to reduce particle size before mixing or formulating materials. It is most commonly used in the production of thermoplastics such as polyvinyl chloride (PVC), nylon, polypropylene and polyethylene.

Generally, in comparison to other technologies, cryogenic grinding systems can increase throughput whilst maintaining the same particle size distribution. They can also achieve finer grinding at the same throughput, although this narrows the production particle size distribution. With either option, there are clear advantages to cryogenic grinding.

Focus on Particle Size

With the increase in demand for higher performance plastics, the industry has been searching for more efficient, high-tech grinding solutions that can produce even finer polymer particles that are distributed more evenly. All this needs to be achieved whilst also maximising throughputs.

For example, the food packaging industry requires the production of plastic films that are ultra-fine but also protective enough to keep foods well-contained and fresh whilst resilient enough to withstand high temperatures when cooking. Using high-tech polymers means that the film is considered to be an integral part of the complete packaging system.

The packaging will extend the shelf-life of the food product when combined with the right food container and mix of atmospheric gases. Plastic particle size reduction is increasingly important for this kind of advanced film application and it helps to ensure that the end product film is light weight and delivers a high performance.

Selecting a Method

Dry milling is one of the most common methods of particle size reduction for the plastics and rubber industries. This method uses a “roll” to compress and yield particles of different sizes so that they can be separated and sorted more easily. However, dry milling alone is not always enough to achieve the ultra-fine particle size required for more complex and variable applications which use the toughest polymers.

Controlled crystal growth or high-density jet milling are other examples of particle size reduction technologies. Similar to dry milling, these processes can be easily scaled up, but they do not necessarily deliver the throughputs and ultra-fine particle distribution required by some processes.

The plastics industry continues to explore cryogenic grinding as the demand for size reduction increases to particles sizes of less than 45 µm (325 mesh). These systems achieve uniform particle distribution and ultra-fine particle sizes, whilst simultaneously minimising overall operational costs and maximising production rates.

The Cryogenic Alternative

Amorphism is a material phenomenon in which there is no long-range order of the molecules in the compound. Amorphous materials exist in two distinct states: glassy or rubbery. Amorphism is the basis for cryogenic grinding, as applied in most industrial environments. Such behaviour can be observed using a thermal scan by an instrument such as a differential scanning calorimeter (DSC).

Fine grinding is possible with cryogenic grinding systems. These systems achieve ultra-fine particle sizes and uniform particle distribution, while maximizing production rates and minimizing overall operational costs.

The DSC identifies different properties of the material, including the temperature where the material transitions between the glassy and rubbery states. This point is commonly referred to as the glass transition temperature (T_g). Therefore, the purpose of the cryogenic fluid in dry milling is to keep the temperature below the glass transition temperature, or in the glassy state. In this state the material is brittle and prone to disintegration.

Hammering a piece of glass at room temperature will break it, whereas hammering a piece of rubber will not. The rubber momentarily deforms or stretches, so absorbing the energy. However, when submerged in liquid nitrogen, the same piece of rubber will behave like brittle glass and become easy to shatter with the hammer. This is because rubber that is cooled by liquid nitrogen is below its glass transition temperature.

The term ambient grading, used in the context of plastics particle reduction, applies to systems where the starting material is fed to the grinding mill at, or slightly below, the ambient temperature. In cryogenic grinding, the starting material temperature is reduced to well below -100°C just before grinding. A cooling conveyer must be specified in order to apply the cryogenic fluid.

The cooling conveyer is operated in a closed system, often vacuum jacketed to reduce heat losses. This primarily provides mixing and resistance time in order to effectively lower the temperature of the material to below its T_g.

Within the enclosed cooling conveyer, the liquid nitrogen is sprayed directly onto the product. In order to maintain a material setpoint temperature as measured at the conveyor (or in some cases at another point in the process), the flow of liquid nitrogen to the conveyer is adjusted. Cryogen consumption, unlike ambient grinding, contributes an additional operating expense and this must be taken into account in the final product costs.

There is little difference in terms of equipment, other than the cryogenic cooling conveyer, between ambient and cryogenic grinding. However, materials and other processing fluids need to be compatible with cryogenic temperatures as well as the material that is to be processed.

Some mechanical milling systems that are commonly used include the attrition mill, hammer mill, turbo mill, and pin mill. All of these systems use a combination of close clearance and high speed to effect particle size reduction through impaction and attrition.

• The hammer mill uses a screen of various hole sizes to maximize the residence time in that grinding zone until the particle has reached the desired size.
• The attrition and turbo mills do not use screens, they depend instead on the gap width between the rotating and stationary parts of the mill in order to control the resultant particle size.
• Similarly, the pin mill does not use a screen to control particle residence time. However, unlike the attrition and turbo mills, it uses two opposing, rotating surfaces with tightly spaced “pins” to drive a reduction in particle size.

In order to meet the specific needs of the production processes, advanced cryogenic grinding solutions use a combination of size reduction mechanisms including attrition, impact, and particle-particle collision. These systems are highly flexible and so can be easily adjusted to help regulate the size of the particles. For example, ultra-fine particles can be separated out by adjusting the grinding clearance to the narrowest setting. This improves the overall efficiency of the grinding system.

Processors are increasingly choosing to test alternative technologies before making a capital investment and selecting a new grinding system. The manufacturer can use advance-testing services to enable them to compare grinding technology systems and determine the best solution for a particular product.

It is possible to test whether the required production rates and particle size distribution can be achieved under laboratory conditions. Insights into how the technology will scale up and the required consumption of liquid nitrogen can also be achieved with laboratory tests.

With cryogenic grinding, the starting material temperature is reduced immediately prior to grinding. Also, to apply the cryogenic fluid, a cooling conveyor must be specified. The cooling conveyor is operated as a closed system that primarily provides mixing and residence time to effectively lower the temperature of the material to below its glass transition temperature.

Looking Ahead

In order to develop solutions that are more efficient and effective, providers of cryogenic equipment continue to advance the capabilities of cryogenic grinding systems. One particular area of research focuses on demonstrating the operational efficiency of cryogenic grinding systems. For example, it aims to illustrate that using liquid nitrogen in conventional cryogenic grinding systems can enhance the overall energy efficiency of the system and produce finer particles as a result. 

With continued research into this area, cryogenic grinding is gaining recognition as a sophisticated, yet efficient, alternative that is capable of meeting the increasingly complex process demands of the modern plastics industry.

This information has been sourced, reviewed and adapted from materials provided by Air Products.

For more information on this source, please visit Air Products.